In practice, many examples of a device having surfaces to be kept clean from biofouling exist, including devices intended for use in a domestic context, such as coffee makers, water disinfectors and boilers, and also including devices intended for use in an industrial context or a marine context, such as box coolers. In general, biofouling of surfaces is a well-known problem, which particularly occurs in the context of surfaces which are exposed, at least during a part of their entire lifetime period, to water or another fluid in which biofouling organisms are present.
Biofouling or biological fouling is the accumulation of microorganisms, plants, algae, small animals and the like on surfaces. According to some estimates, over 1,700 species comprising over 4,000 organisms are responsible for biofouling. Hence, biofouling is caused by a wide variety of organisms. Biofouling is divided into micro biofouling which includes biofilm formation and bacterial adhesion, and macro biofouling which includes the attachment of larger organisms. Due to the distinct chemistry and biology that determine what prevents them from settling, organisms are also classified as being hard or soft. Hard biofouling organisms include calcareous organisms such as barnacles, encrusting bryozoans, mollusks, polychaetes and other tube worms, and zebra mussels. Soft biofouling organisms include non-calcareous organisms such as seaweed, hydroids, algae and biofilm “slime”. Together, these organisms form a biofouling community.
In several situations, biofouling creates substantial problems. Biofouling can cause machinery to stop working, water inlets to get clogged, and heat exchangers to suffer from reduced performance. Hence, the topic of anti-biofouling, i.e. the process of removing or preventing biofouling, is well-known. In industrial processes involving wetted surfaces, bio dispersants can be used to control biofouling. In less controlled environments, biofouling organisms are killed or repelled with coatings using biocides, thermal treatments or pulses of energy. Nontoxic mechanical strategies that prevent organisms from attaching to a surface include choosing a material or coating for causing the surface to be slippery, or creating nanoscale surface topologies similar to the skin of sharks and dolphins which only offer poor anchor points.
Box coolers are heat exchangers comprising a plurality of pipes extending at quite a close distance with respect to each other, which are especially intended for use in engine-driven marine vessels such as ships. Normally, a ship is equipped with various kinds of machinery, and one or more box coolers of which at least the pipes are arranged in one or more sea chests may be used in a machinery cooling system of the ship. The pipes of a box cooler serve for containing and transporting fluid to be cooled in their interior, wherein it is a practical option for a sea chest accommodating the pipes to have both inlet openings and outlet openings so that water can enter the sea chest, flow over the pipes in the sea chest, and exit the sea chest through natural flow and/or under the influence of motion of the ship. Biofouling of a box cooler is a major problem in view of the fact that layers of biofouling are effective heat insulators, so that biofouling involves a decrease of the heat transferring capability of the box cooler. When the biofouling layers are so thick that seawater can no longer circulate between adjacent pipes of the box cooler, an additional deteriorating effect on the heat transfer is obtained. Thus, biofouling of box coolers increases the risk of engine over-heating, so that ships need to slow down or damage of ship engines occur.
For example, in the case of a tugboat having an installed engine power of 15 MW, one or more box coolers are applied for transferring heat in the order of 5 MW to the seawater. Usually, a box cooler comprises bundles of U-shaped pipes for conducting a fluid to be cooled, wherein ends of leg portions of the pipes are secured to a common plate having openings for providing access to both leg portions of each of the pipes. It is a very practical option to enable the box cooler to perform its cooling function by continuously exposing the pipes thereof to fresh seawater, as mentioned in the foregoing. However, the environment of a box cooler is ideally suited for biofouling, as the seawater is heated to a medium temperature in the vicinity of the pipes as a result of the heat exchange with the relatively hot fluid in the interior of the pipes during operation of the box cooler, and the constant flow of water continuously brings in new nutrients and organisms which are known to cause biofouling.
Anti-biofouling arrangements for cooling units that cool the water from a cooling water system of an engine-driven ship by means of seawater are known in the art. For example, DE 102008029464 relates to a box cooler for use in ships and on offshore platforms, comprising an integrated anti-biofouling system for killing biofouling organisms by means of an overheating process that can be regularly repeated. In particular, the box cooler is protected against microorganism biofouling by continuously overheating a defined number of heat exchanger pipes without interrupting the cooling process, wherein waste heat from the cooling water may be used for doing so.
Plate coolers are heat exchangers comprising plates in a successive arrangement, and are typically used for enabling a transfer of heat between two liquids. The plates are normally made of metal or another material which is known for having high thermal conductivity. In a plate cooler, the liquids are spread out over plates, so that it is possible to have a relatively large heat exchanger area and still have a compact overall construction. A widespread application of plate coolers is an application in combination boilers, for example, which does not alter the fact that application of plate coolers in an industrial context is also common. As the plates of a plate cooler are exposed to liquids throughout the lifetime of the plate cooler, biofouling of the plates occurs, which involves a reduction of the heat transferring capability of the plate coolers, and which may eventually lead to failure of the plate coolers, hindering the necessary flows of liquid through the plate coolers to a too high extent. This problem is all the more apparent when the plates are corrugated, which is often the case in practice, as having such plates is a way of realizing a further enlargement of the heat exchanger area. In the art, methods of cleaning plate coolers are known in order to alleviate the problems caused by the biofouling phenomenon, which methods include taking the plate coolers apart and cleaning the plates one by one.
In general, it is known in the art to use ultraviolet light for removing/preventing the formation of biofilm on wet surfaces. For example, WO 2014/014779 discloses a system for reducing fouling of a surface of an optically transparent element subjected to a marine environment, including a LED for emitting ultraviolet radiation, a mount for directing emitted ultraviolet radiation toward the optically transparent element, and control circuitry for driving the LED.
The invention is particularly relevant in respect of devices having surfaces which need to be kept clean from biofouling yet are not transparent to the rays of anti-biofouling light emitted by the anti-biofouling system during operation of the at least one anti-biofouling light source thereof. A problem associated with many conventional assemblies of a device having surfaces and an anti-biofouling system comprising at least one anti-biofouling light source for emitting rays of anti-biofouling light is that one or more of the surfaces are in the shadow with respect to the rays of anti-biofouling light from the at least one anti-biofouling light source, so that biofouling still occurs at those surfaces. This problem is especially apparent in case the device has many surfaces in a complex arrangement. For example, in the context of a conventional box cooler having an anti-biofouling system which comprises at least one anti-biofouling light source for emitting rays of ultraviolet light, it appears to be practically impossible to have the anti-biofouling effect as desired on the entire exterior surface of all pipes, because in a normal situation in which only a limited number of anti-biofouling light sources can be added to the box cooler, it cannot be avoided that pipes are in the way between other pipes and the one or more anti-biofouling light sources. Even if one or more anti-biofouling light sources are arranged at a position between pipes of the box cooler, no more than a possibility of increasing a total area of the surfaces to be kept clean from biofouling is realized, while the fact remains that it is not possible to have a situation in which the entire area of the surfaces can be reached by the rays of ultraviolet light.